(1) Field of the Invention
The present invention relates to a device for driving a piezoelectric element which is used, for instance, as an actuator portion of a fuel injection valve applied for an internal combustion engine of an automobile.
(2) Description of the Related Art
It has been known to use a piezoelectric element, for instance, PZT (55% PbZrO.sub.3 and 45% PbTiO.sub.3), as an actuator for a valve such as a fuel injection valve, and a pump such as a fuel injection pump in which the actuator is required to have a certain stroke. In a circuit for driving the piezoelectric element used for such an actuator, it is known to use transistors or thyristors.
If a PZT piezoelectric element is applied as an actuator for a valve or pump, it is necessary to apply a voltage of several hundred volts to the PZT piezoelectric element, which is therefore applied with electric current of several tens of amperes.
If transistors are used for a circuit for such an actuator, the transistors must be of a high-voltage and large-current type, which are very expensive. Thyristors, which were originally developed for use for high-voltage and large current purposes, may be suitable for a circuit driving an actuator including a PZT piezoelectric element. However, when a coil is used for commutating the thyristors, the PZT piezoelectric element is applied with a negative voltage and suffers from adverse effects due to polarization.